The detection of volatile organic compounds (VOCs) present in ambient air is of great concern to air quality monitoring programs because of the potential hazards to human health and the environment but also of interest to forensic scientists interested in detecting VOCs associated with, for example, explosives, drugs, and ignitable liquid residues. The list of VOCs thought to be toxic organic compounds is extensive and has been compiled by the Environmental Protection Agency (EPA) to indicate chemicals of concern that may be detectable in areas where air pollution is present, such as industrial sites. While the severity of the hazard of different contaminants varies, many VOCs present in ambient air have the potential to act as mutagens and carcinogens. Therefore, unequivocal detection and quantitation of VOCs is important to managing and mitigating health impacts from toxic compounds. In an effort to address this issue, the EPA has published the “Compendium of Methods for Toxic Organic Air Pollutants” since 1984 (TO-13 to TO-17). These are a series of reports describing the most current methods and guidelines to be followed for the monitoring of VOCs in ambient air or polluted environments.
The analysis of VOCs in ambient air is currently performed with sorbent tubes following the guidelines from the EPA method TO-17. The commercially available sorbent tubes consist of a thin cylinder that can be made out of glass or stainless steel. The interior of the tubes are packed with sorbent material, thus the name sorbent tubes. Commonly used sorbent materials include: several variations of Tenax®, Carbotrap®, and Carbopack™, with the possibility of using multiple sorbents in a single tube. The sorbent material selected depends largely on the target compounds, specifically the volatility or vapor pressure of the molecule of interest. Each sorbent material is classified according to its strength, which is described as the affinity of the compounds to the sorbent. A strong sorbent will allow greater sampling volumes for all or most of the targeted VOCs and the strength of a sorbent tube is related to the surface area of the sorbent material. A weak sorbent has a surface area less than 50 m2/g (e.g. Tenax® TA), a medium sorbent has a surface area in the range of 100-500 m2/g and a strong sorbent has a surface area around 1000 m2/g. In general, stronger sorbents are used for highly volatile compounds.
Some of the limitations observed for the analysis of VOCs with sorbent tubes include: long headspace extraction times (˜1 hr) with low flow rates and the use of expensive thermal desorption units coupled to gas chromatography-mass spectrometry (GC-MS) through the use of transfer lines that can result in poor recoveries.
A capillary microextractor of volatiles (CMV) device, which can be a tube, is an extraction method for the detection of VOCs in forensic and environmental applications, as taught in Almirall et al. U.S. Pat. No. 9,267,866 Feb. 23, 2016. The CMV method has been demonstrated to result in improved sensitivity and selectivity compared to SPME, for the extraction of volatiles in the headspace of smokeless powders. Furthermore, it has also been shown to be effective for the detection of gunshot residues on swab samples from the hands of shooters. A standard CMV consists of an open-ended 2 cm glass capillary tube packed with vinyl terminated polydimethylsiloxane (PDMS) coated glass filters, as shown in FIG. 1. The inner diameter of the glass capillary is 2 mm and is packed with approximately seven strips (2 cm×2 mm) of the PDMS coated glass fibers. PDMS is a non-polar polymer that provides a hydrophobic coating over the glass filter, thus improving extraction of VOCs in humid conditions.
The use of PDMS has been demonstrated to be more effective than Tenax® TA for the analysis of large injection volumes and retention efficiency with good precision. With a total surface area of ˜0.05 m2 in the sampling device and a phase volume of 100 mm3 for the CMV, the device offers greater absorption capacity compared to SPME where the surface area is 10−5 m2 and phase volume is only 0.612 mm3, improving the capacity of CMV over SPME by 5,000 times. It also facilitates dynamic sampling, thus increasing sampling efficiency and decreasing sampling time. A method to further enhance the efficiency of CMV is desired. Yet, further improvements to the degree of volatility and the quantity absorbed is desirable.